<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>16:9的盒子</title>
    <style>
      *{
        margin: 0;
        padding: 0;
      }
      .box {
        --width: 375px;
        width: var(--width);
        height: calc(var(--width) * (16 / 9));
        background-color: lightblue;
      }
    </style>
  </head>
  <body>
    <div class="box"></div>
  </body>
</html>
